Not all hobs are created equal.

Many hobs must be designed before production, and gear manufacturers who seek the most from their cutting tools choose suppliers that not only design but also engineer their hobs. An engineered hob considers the entire hobbing application and can optimize a rough or finishing hobbing process. Consequently, the hob increases productivity, provides longer tool life, and reduces cost-per-piece.

Expert hob manufacturers request at least a part print before hob design. Even better, the hob supplier will collect information about the entire hobbing application. With this detailed information, the hob manufacturer can engineer specifications that positively impact the gear hobbing application. Without a drawing and other important application information, or if the manufacturer merely provides “generic” hobs, decisions on hob design are made for an average case. Thus, if an optimized hobbing application is the goal, an average hob design will likely fall short in competitive performance.

An engineered hob costs more than a generic hob, and that amount can be relatively large: up to 50% more. However, the overall performance of the hobbing application should be analyzed to understand the true financial impact and process time effects of an engineered hob. Consider the following simplified example:

This example illustrates that at a 50% increase in hob cost may be recouped after cutting only 427 parts. For medium- to high-volume jobs, an engineered hob is obviously preferred, provided it delivers on its promises. Consider, too, that shorter cycle times mean quicker turnarounds and increased machine time availability. Today’s top gear manufacturers know this is one key to remaining competitive.

Engineering the Hob

What aspects of a hob’s design can be engineered? Consider the hob’s material. Not only are there a variety of high-speed steels available today, but the same is true for carbide. By knowing the exact hobbing application (work piece, machine, clamping strategy, automation, cutting process, pre- and post-operations, etc.), the tool manufacturer can optimize the choice of hob material. Moreover, because tool materials require heat treatment, even this process can be optimized. Consequently, the tool’s hardness, toughness, and other physical properties may provide improved wear resistance (tool life) and cutting speeds (productivity).

Figure 1 – Specific choice of material can optimize hob performance

For pre-finish hobbing, the tool designer may also study the exact finishing process that will be used. Accordingly, an optimized hob tooth profile for the roughing operation can be engineered. This may improve rough hobbing performance, but it can also benefit the finishing process. For example, the pre-finish hob can be engineered to hob an optimal stock on each tooth flank for the specific application’s work piece material, tooth geometry, and heat treatment process. This can lead to extended hard finishing tool life and/or shortened hard finishing cycle times.

Figure 2 – Engineered pre-finishing tooth profile

Similar thinking can be applied to a re-hobbing finishing process. Furthermore, this type of hob engineering can impact design choices for coating, profile and lead modifications, thread starts, number of flutes, tooth relief, hob grinding method, and more. To make the most of a hob, the gear manufacturer should fully discuss all possible parameters with the tool manufacturer.

Figure 3 – Helios Ra-Cut hobs offer optimized flute designs

In conclusion, a tool manufacturer can engineer an optimized hob for measurable improvements in productivity. However, this engineering requires the tool manufacturer to consider the full context of the specific hobbing application. It also requires a tool manufacturer with the appropriate level of expertise to deliver a hob’s full potential. Consequently, gear manufacturers can open new doors for their production to remain competitive for the future.

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An introduction to the hard finishing of gears.

In Part 1 of the Hard Finishing of Gears, we introduced the concept and a brief history of processes used to achieve non-hardened finished parts. Continuing, consider a gear that has been pre- or semi-finished, meaning additional (“plus”) material has been left on the flanks of each gear tooth and, in some cases, in the root area as well. After this part has been heat treated, it must undergo hard finishing, which removes a small amount of stock to arrive at the final, geometrically accurate profile, lead, and spacing. Gear manufacturers today use a variety of finishing processes for this purpose (see Figure 1).

Contemporary CNC gear hobbing machines often have the capability for carbide rehobbing. Put simply, this operation is a carbide re-hobbing process for hardened gears that accurately removes stock from both flanks of gear teeth. This is achieved via synchronous timing between the work and cutter spindles after orienting the work piece. (For more information on this process, see our previous article.) Carbide rehobbing, sometimes called “skiving,” removes errors such as cumulative pitch, which cannot be achieved with tool-driven finishing methods such as those discussed in Part 1. Generally speaking, carbide rehobbing can achieve AGMA Q8 – Q11 quality, which is limited by the generating process of hobbing and the number of gashes of the hob.

“Skiving” can be an ambiguous term. Along with “scudding” and “power skiving,” the term can refer to a process that uses a disc-like cutter and work in a crossed-axis relationship. This allows continuous generation for both internal and external gears. For internal gears where the hobbing process is not practical, this skiving process offers significant speed advantages compared to traditional shaping. It is mentioned here because there are machines available that use this process to finish a hardened gear.

To achieve even greater quality, a gear manufacturer must turn to finishing processes with an undefined cutting edge. Grinding serves this purpose, where the “cut” is made by each grain of a grinding wheel instead of a defined tooth. Form grinding, also called profile grinding, where the tool takes the conjugate shape of the work piece, is a process where each tooth space is ground individually. It can be used for both internal and external gears. Generating grinding is akin to hobbing in that the tool takes the shape of a worm that mates with the work piece. A timed relationship between the work and grinding spindles allows accurate stock removal from the hardened gear. Grinding can achieve AGMA Q12 – Q15 quality, but this process requires special machinery designed around the process, tools, and work pieces.

Honing is an alternate hard finishing process that uses an abrasive tool with an undefined cutting edge like traditional grinding. This process uses a dressable wheel (also called a ring or stone) that meshes with and envelopes the work piece. Designed to be in a cross-axis relationship, the work piece is put under pressure from the honing wheel. Once these two components turn in mesh, a sliding, shaving action removes material. This process can achieve AGMA Q10 – Q13 quality.

Finally, the gear manufacturing industry will soon see more options for “superfinishing.” Whereas hard finishing processes remove hardened stock material to arrive at a finished gear geometry, superfinishing focuses on achieving better surface finish quality. For example, in a polishing process, a gear manufacturer may use a continuous generating wheel of a unique material to improve gear teeth surfaces to mirror-like finishes. The AGMA does not currently specify standards for surface finish.

The hard finishing of gears has become a common process for gear manufacturers. Hardening gears increases life and resistance to handling damage, but the heat treating distorts tooth geometry. Thus, pre-finishing processes typically leave stock material along tooth flanks, and hard finishing processes remove this material after heat treatment to achieve a geometrically accurate tooth. Finally, concurrent or subsequent superfinishing processes may also increase surface finish quality.

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An introduction to the hard finishing of gears.

Generally, many “hardened” gears in the United States before the 1970s were a through-hardened part, but this changed to contemporary case-hardening processes that were considered relatively new technology at the time. This movement in the 70s included a carburizing process that mirrored the success already achieved in Europe, and it granted a tougher case of 50-60 HRC or higher on the surface. Hardening of gears grants many benefits, such as improved life and resistance to handling damage, but it requires special hard finishing processing consideration to achieve a high geometric quality part. Note that gear “quality” usually refers to geometric specifications and tolerances such as that set out in ANSI/AGMA ISO 1328-1-B14, but in other contexts it could also refer to surface finish or metallurgical quality.

Hardening processes tend to distort gears, especially helical gears because these want to unwind as a result. Hard finishing is a final machining process after hardening that results in a geometrically accurate gear. It removes distortion in addition to any minor handling damage. Furthermore, most hard finishing processes can also improve the surface finish of a gear.

For reference, consider green (non-hardened) finishing processes that were used through the 1980s and some still today, such as shaving, lapping, burnishing, and rolling. Shaving requires a special tool that rolls a part under pressure in a crossed-axis configuration to remove stock. Lapping is another process that requires the tool to drive the work piece, and this driving makes it impossible to completely correct all gear teeth errors. Burnishing uses a sliding action over the entire active profile, which rubs to refinish the tooth surface and is not a process for removing considerable stock. Finally, finish rolling displaces green material to arrive at a finished part, and as such it cannot be used for hardened parts.

Due to the limitations of the above processes, the gear industry developed a variety of contemporary methods for hard gear finishing such as grinding, skiving, honing, and polishing. (Some would refer to the latter as a “superfinishing” process, which we will discuss later.) By using a precise synchrony between a finishing tool and a work piece, errors such as accumulative pitch can also be removed. In comparison, tool-driven methods cannot achieve this. The development of computer numeric controlled (CNC) gear machines allowed such synchronous processes to be easily attained with quick, flexible setups. Consequently, these processes have become common for gear manufacturers today.

Contemporary hard gear finishing typically starts by green cutting, which in this case may be referred to as semi- or pre-finishing. This process cuts the non-hardened gear teeth such that they have plus material along the flanks and often with undercut from a protuberance-type hob. After heat treatment, a hard finishing process then removes the (distorted) plus material, thus blending the final profile with the undercut.

Semi-Finishing Hob, Gear with Protuberance

Gear Tooth Showing Pre-Finishing Stock

In our next issue, we will introduce the details of skiving, carbide re-hobbing, form grinding, continuous generating grinding, honing, and polishing. Additionally, we will provide an estimate of achievable AGMA gear qualities per process.

An introduction to coatings and their use for gear cutting tools.

In this issue, we will discuss one of the most important aspects of gear cutting tools aside from the tool itself: tool coatings.

TiN-coated Hob

The most common process through which coatings adhere to tools is called Physical Vapor Deposition (PVD). Before the PVD process, tools are physically and chemically cleaned thoroughly. This ensures no contamination alters the coating and allows a consistent, measurable process. During PVD, the tools become the cathode of a high voltage circuit in a reaction chamber. Argon gas is injected into the chamber to further clean the tools. A metal ingot, usually titanium, is heated with an electron beam until it evaporates. A reactive gas (nitrogen in the case of TiN) is then injected into the chamber and is electrically accelerated toward the tools. The gas combines with the evaporated metal, creating the chemical composition of the coating that adheres to the tools. This process operates within the range of 900°F, which is well below the tempering range of high speed steel (HSS) and keeps the tools from being softened and rehardened. The end result is a tight adhesive bond with a coating that is as thin as 1-4 μm, allowing for sharp edges to be retained.

Tool coatings have been available since the 1970s when Titanium Nitride (TiN) was introduced. TiN is still the most popular coating for cutting tools in general, though advances in coating technology allow manufacturers today to choose from many different options and reap a variety of benefits for gear cutting applications. Compared to common cutting tools such as end mills, gear cutting tools are costly, so the choice of coating can provide important cost savings.

AlCrN-coated Hob

There are many benefits from coating cutting tools, such as reduced wear on the tool. This is achieved by acting like a non-stick coating. Coatings prevent direct contact of chips with the tool, reduce buildup along the cutting edge, and avoid welding of chips to the tool.

Coatings have a high hardness, which prevents abrasive wear on the tool. They also have a lower coefficient of friction than most cutting materials, which reduces heat and wear generated during cutting. The thermal conductivity of coatings is lower than that of cutting materials, which allows heat to be retained in the chip, reducing undue thermal stress on the tool. Together, these attributes can improve tool life by 200–300% and in some applications allow for an increase in speeds and feed rates by as much as 30–50% compared to similar uncoated tools.

There are three leading coatings that are in use for gear cutting applications today: TiN, Titanium Aluminum-Nitride (TiAlN), and Aluminum Chromium Nitride (AlCrN).

TiN

TiAlN

AlCrN

Microhardness (Vickers)

2300

3300

3200

Coefficient of Friction

0.4

0.30–0.35

0.35

Maximum Service Temperature

600 ℃

900 ℃

1100 ℃

All of these coatings can be used in applications which use coolant. Some dry hobbing applications can use TiN, but TiAlN and AlCrN have better success due to their higher maximum service temperature (see table above). These coatings have all-around desirable qualities, with TiAlN and AlCrN being the current forerunners in gear cutting applications.

Coating technology is constantly being upgraded and tested, so gear manufacturers should stay in the loop and be cognizant about updates that take place in the coating world. For example, Oerlikon Balzers (a globally prominent coating solution provider) recently announced their newest generation of gear tool coating, which they claim provides significantly increased cutting performance over traditional AlCrN. The correct choice of cutting tool coatings can greatly increase the productivity of gear cutting applications.

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The effects and causes of hob flute lead error.

In previous issues, we looked at several types of hob errors: flute spacing error, rake error, and runout. Another important type of error can be found in the lead of the flute, also known as the gash. Flute lead error occurs when the flute deviates from the correct flute helix angle. For straight-gash hobs, this is when the flute is not consistently parallel with the axis of the hob. Besides incorrect original manufacturing, hob sharpening is typically the cause of this type of error. Ahead, this is explored in more detail as well as the consequences of cutting a gear with a hob with this error.

A hob with flute lead error will have one end that has had too much stock removed compared to the other end. This creates a hob with a tapered outside diameter. As this tool is used to cut gears and shifted through its useful length, size variation will occur in the produced parts. Extreme taper will generate gear teeth that are unsymmetrical. They will have involute profiles that are plus (+, positive) on one side of the tooth and minus (-, negative) on the other, which creates a leaning profile.

Because only a small portion of a hob’s length is used to generate the gear form, inspection of a single gear is unlikely to highlight the effects of flute lead error. Therefore, by comparing measurements of gears cut at the start and end of a complete hob shift, one should be able to detect any potential problems in the lead of the hob flute. In particular, a difference will be apparent in tooth thickness and depth.

What causes flute lead error? Assuming the original manufacturing of the hob was correct, flute lead error is caused by poor hob sharpening or a poor hob sharpening machine and/or setup. Hob teeth are form-relieved, so if the flutes are sharpened at an incorrect angle, all of the teeth of the hob will be adversely affected. Too much stock will be removed from one end of the hob, creating teeth that are thinner and extend to a smaller diameter than the other end. This creates the tapered outside diameter of the hob that was discussed above. Specifically, taper will be induced by sharpening if the headstock and tailstock are out of alignment. Also, an incorrect sine bar setting or incorrect lead change gears (if the machine is equipped with either) can also induce flute lead error.

The images below show a hob flute being inspected for taper along the faces of a single row of teeth.

Hob Flute Lead Measurement

The AGMA standard ANSI/AGMA 1102-B13, Tolerance Specification for Gear Hobs, sets forth criteria for flute lead error tolerances. For example, over 100 mm length, the deviation from the correct lead must be less than 40 µm (0.0016”) to achieve an AGMA “AAA” quality. Details on these qualifications can be found in Table 8 – Accuracy Requirements of the standard, specifically Test no. 7, shown below.

AGMA 1102-B13 Table 8, Test 7 – flute lead check

When gear tooth size variations occur over the complete shift of a hob, flute lead error is likely a cause. This can be confirmed by measuring the deviation of the flute from the correct lead, or by observing a tapered outside diameter of the hob. This error is typically caused by poor sharpening and can be corrected in two ways. First, ensure correct alignments in the sharpening machine, and second, confirm correct sine bar settings and lead change gears if used.

A look at the first K160 Repowered CNC gear hobbing machine

Koepfer America has completed the next step in its K-Repowered CNC re-control program with the first K160 Repowered CNC Gear Hobbing Machine, which was displayed at Gear Expo 2015 in Detroit, MI. New, potential customers were impressed with the quality of the machine as a cost-effective, like-new hobbing machine, and current customers were interested to see proof of the K-Repowered package as a new option to extending the life of their existing Koepfer Model 160.

Koepfer America K160 Repowered CNC Gear Hobbing Machine

The K-Repowered Re-Control Package provides the natural solution to future-proofing existing Koepfer Models 160 and 200 for another 20+ years with Fanuc CNC. Of the hundreds of North American Koepfer machine installations, roughly half are models 160 and 200. An estimated half of those are of a vintage where the K-Repowered program should be considered by owners.

After a K-Repowered Re-Control, a Koepfer Model 160 or 200 is certified and warrantied by Koepfer America. It essentially becomes a like-new machine with new and improved electronics, motors, software, controls, etc. As a simplification, all existing Koepfer Models 160 and 200 are categorized as either too new to consider the K-Repowered package or as one of the following. With a “Level 1” re-control, the machine is mechanically sound, but it would benefit from a K-Repowered Re-Control. In a “Level 2” re-control, the machine would need mechanical repair in addition to theK-Repowered Re-Control. Koepfer America sends trained, certified technicians to assess which level is the best solution for the customer’s Koepfer Model 160 or 200.

A Level 1 re-control will cost approximately 1/3rd the cost of a brand new machine, whereas a Level 2 cost will vary more substantially around 2/3rds the cost of a new machine. New equipment may cost more than a K-Repowered Re-Control. Level 1 re-controls are designed to be performed in the field in a matter of weeks. This minimizes machine downtime while Koepfer America’s engineers and technicians complete the re-control. For Koepfer Models 160 and 200 that are 20+ years old (especially those that have been run hard), the K-Repowered Re-Control Package is an economic alternative compared to the full purchase of new equipment.

Koepfer America has always focused on customer service and being a partner with gear manufacturers. The development of the K-Repowered program demonstrates the dedication to this mission. As Koepfer America’s machine install base continues to grow, it also continues to age. Gear manufacturers rely on Koepfer to ensure the longevity, effectiveness, and uptime of their Koepfer CNC gear hobbing machines. They can rest assured that their Koepfer Models 160 and 200 can have several decades ahead to continue providing industry-leading flexible gear cutting, automation, and some of the easiest-to-use software and ergonomic machine designs.

The benefits and use of automation during the hobbing process.

For this discussion, we will categorize parts into three groups: large, medium, and small. Large parts above 400 mm (~16 in) are typically not automated. As a general rule, parts that are 200–400 mm (~8–16 in) diameter can be considered medium sized and are normally processed on vertical hobbing machines. Parts less than 200 mm (~8 in) diameter can be considered small and can be efficiently automated on both vertical and horizontal hobbing machines.

What exactly is “automation”? Gear hobbing automation is a system that integrates with a machine tool to automatically load and unload blanks and gears through the use of hydraulic and pneumatic cylinders, grippers, claws, etc. (see the video above). The first generation of automation for gear cutting was dedicated, engineered systems which were appropriate only for mass production due to their cost and difficulty to design and implement. “Blue Steel” was used to convey parts into and out of the machines. (Figure 2) For larger parts, such as starter ring gears, other systems were developed. (Figure 3) However, none of these designs provided flexibility to accommodate several different work piece types.

Figure 2 – Blue steel used for mass production automation

Figure 3 – Automation of larger parts

The second generation systems typically used pallets to transport gear blanks on a moving chain loop storage. (Figure 4) These dedicated systems were designed and optimized specifically for the production of a single part with large lot sizes. For example, automotive applications may implement automation to decrease machine idle times to 5 – 7 seconds. This may seem insignificant, but when it is multiplied over thousands of parts on several machines, the benefits become significant.

Today’s flexible automation systems can adapt to a larger number of part types. These systems can be set up in less than 30 minutes, allowing lot sizes as small as 25 pieces to be run with the benefits of automation. This is one key to a successful gear hobbing process. For example, Koepfer gear hobbing machines are used and recognized throughout the world due to their flexible automation. They can accommodate pinions, shaft type parts, as well as bore types using one basic automation system. The Koepfer machines are horizontally-configured and have parallel axes for cutting and automation, which allows gravity to aid the loading and unloading of parts. This reduces the complexity of the automation (less cost and easier changeover) and reduces motion (faster cycles). In the end, flexible automation provides the critical ability for job shops to minimize their work-in-progress costs, decrease labor needs, decrease cycle times, and produce parts more consistently. (Click here to learn more about Koepfer automation.)

Figure 4 – Automation with pallets

Despite the additional upfront cost of an automation system, it will increase the efficiency of machines, reduce labor costs, and increase machine utilization. Over the life of a machine, these gains will easily outweigh the costs. Therefore, automation for gear hobbing machines provides an invaluable means to getting the most from the machine tool, which leads to greater productivity.

Figure 5 – Side and Top Views of Automation of Pinions

Figure 6 – Side and Top Views of Automation of Bore-type Parts

In gear manufacturing, the use of automation should be considered to help achieve a highly productive and profitable operation. With each new generation of hobbing machines, the speed and flexibility of the automation systems also improve. Automation brings many advantages. The greatest is the elimination of human operation “interference” time, and it also increases machine utilization because less time is spent loading work pieces. The operation will also be engineered, which provides more consistently accurate and precise parts.

A detailed look at gear tool resharpening.

In Issue 13, we discussed the importance of tool quality and the proper maintenance of a tool that shows signs of wear. In this issue, we will cover the basics of the process used to sharpen the hob.

The types of wear on a cutting tool can fall into a number of categories, all of which are undesirable and can reduce the quality of cut gears. To ensure that a tool cuts accurate gear teeth, it is necessary to remove any material that shows signs of wear or damage. This takes us to the first step of sharpening, which is to determine the amount of material for removal.

Figure 1 – Examples of typical hob wear.

A cutting tool must be evaluated under a microscope so that even minute amounts of wear can be observed and factored into the sharpening process. While in most cases one wants to remove all damaged material, factors such as cost and preservation of tool life also need to be considered. When a tool displays excessive amounts of wear that would require removing more than 20% of the overall tool life, more discerning sharpening should be used. A minimum amount of material should be removed to render the tool with an optimal number of adjacent, usable convolutions of cutting teeth. This “optimization” requires a skilled, experienced technician to balance sharpened convolutions with decreased tool life. In the end, this approach helps maximize tool life while still allowing the tool to effectively cut accurate gears.

Figure 2 – Example of excessive wear.

Once the operator has determined how much material should be removed, a grinding wheel should be selected according to the material and size of the cutting tool. Typically, CBN (cubic boron nitride) grinding wheels are used for high speed steel tools and diamond grinding wheels are used for carbide tools. The geometry of the wheel should be selected to fit the depth and space of the tool’s flutes while maximizing the wheel’s rigidity.

It is important to ensure that the work-holding is accurate and held within tolerance. If there is any runout present in the work-holding, it will translate to the tool and will result in errors being sharpened into the tool.

Once the tool has been properly mounted onto the correct work-holding and placed in the sharpening machine, the operator must choose the appropriate sharpening program for the tool being sharpened. They must then input all data relating to coordinates, feed rates, spindle speed, and relevant information regarding the tool. It is crucial that the operator understands how each piece of data affects the sharpening process.

During the sharpening process, it is important to intermittently true, or dress, the grinding wheel so that it remains “open.” This term refers to keeping sharp grains of the grinding wheel exposed. Inadequate dressing and truing of the grinding wheel can generate unnecessary friction against the tool and create burn marks. For harder cutter materials, it is often recommended to use softer grinding wheels. Softer wheels dull and break down easier, which means that they are quicker to expose the underlying, sharper grains of abrasive material. This in turn reduces the chance of burning, but it requires more frequent dressing. It is vital to use a high quality diamond when dressing a grinding wheel.

Once the sharpening process has been completed, the operator examines the tool to confirm that all undesirable wear has been removed. At this point they also confirm that the rake error, flute lead error, and spacing are within the tolerance set forth by the latest revision of AGMA standard 1102 and the tool’s quality specification.1

Once the tool has been sharpened successfully, it must be deburred if needed. It is then ready for additional processing (e.g. coating) or set up for cutting again.

— Written by Keith Eller, Application Engineer.

1. For more information, see ANSI/AGMA 1102-B13 “Tolerance Specification for Gear Hobs”

Hobs mounted without precision.

Consider the following two gear tooth profile inspection charts (these have been simplified for illustrative purposes from what one would see in practice):

Figure 2 – Profile Chart Showing Negative Tip Condition

Figure 1 – Profile Chart, Showing Positive Tip Condition

Inspection charts such as these plot the geometric variance normal to a gear tooth’s involute curve on the horizontal axis. The vertical axis is a plot of the position along the involute curve. Knowing this, one can see that neither chart depicts a perfect involute, which would be a vertical line. The charts both show similar error, which is a “leaning” deviation from the involute curve.

Figure 1 shows a positive condition towards the top of the chart, meaning the tooth has more material than it should at its tip. For a hobbed part, this most likely indicates two potential sources of error: poor hob sharpening or improper hob head swivel angle. To address the former source of error, one must check the geometric quality of tool sharpening. In particular, the rake angle of the tool must meet the necessary quality standards. A positive rake error condition of the hob will result in a positive condition of the gear tooth’s involute profile, which is the case in Figure 1. Secondly, the hob head swivel angle must be correctly set. In the case of repeated tooth tip profile error, a hobbing machine’s hob head alignment or scales may need correction.

Figure 2 shows a negative tooth tip profile condition, which actually suggests the tooth has more material than it should at its base. Similar to the positive error condition, this condition has the same potential sources of error. Check to ensure correct position of the hob head swivel. Also, the rake angle of the hob may have been sharpened incorrectly. Check for negative rake angle conditions, which would produce this error. Figures 3 and 4 show the effects of rake error, which are also discussed in more detail in Issue 2, Hob Rake Error.

Figure 3 – Positive Rake Error

Figure 4 – Negative Rake Error

Tooth tip profile error can be corrected with minimal effort. Inspect the cutter for proper rake angle. If rake does not meet the hob’s quality classification then check the sharpening process. Lastly, always check for correct hob head swivel angle.

The importance of tool quality.

The achievable quality of a gear is greatly affected by the quality of the cutting tool used. “Quality” in this context means geometric accuracy (as opposed to metallurgical quality or surface finish, for example). A gear’s geometry can only be as good as the geometry of the cutting tool used to produce it. Therefore, cutting tool maintenance and quality should be a top priority when manufacturing gears.

In a generating process, such as hobbing, the hob (or cutting tool) “meshes” with the work piece to create the profile via the successive cuts of the multiple teeth of the hob. As such, the geometric accuracy of the tool’s cutting edges directly impacts the accuracy of the gear’s profile. Therefore, the original quality level of the cutting tool is of utmost importance when considering tool and gear quality. Regardless of a tool’s maintenance, it is impossible to improve the overall or total quality level of a cutting tool over its original manufactured quality. The American Gear Manufacturers Association sets the standard for determining a cutting tool’s quality.1 One could make the following designations with regard to hob quality levels:

“A” = good quality

“AA” = premium quality

“AAA” = ultra-premium quality

It should go without saying that in order to maintain tool quality, extreme care should be taken when handling cutting tools. This is especially important when handling tools of a relatively brittle material such as carbide. Avoid contact between the cutting tool and any hard surfaces. Simply rolling a hob along a hard tabletop, for instance, can introduce microfractures to the tool’s cutting edges. This will accelerate the wear that the tool experiences during cutting, which will decrease the number of parts (gears) per sharpening.

During use, high quality cutting tools need a high quality mounting or clamping solution. Hob arbors, for example, should be clean and free of any nicks, burrs, or damage. Proper mounting of a cutting tool should have less than 0.0003” radial and axial runout as measured at the hubs and hub faces, respectively. Additionally, the rigidity of the clamping solution should be designed such that minimal vibrations are induced during the cutting process. Without meeting this requirement, the cutting tool will suffer amplified wear, which in turn will require additional resharpening and maintenance needs.

Effects on work piece due to poor mounting.

Once a cutting tool is too worn to continue producing quality gears, it must be resharpened to maintain its quality. In fact, this resharpening process can be thought of as “returning” the tool to its original quality, but only high quality resharpening will return a high quality cutting tool. Thus, the cutting tool resharpening process should be another top priority when producing gears. In a future issue, we will go into greater detail on the requirements for high quality resharpening.

Hob sharpening

To conclude, basic cutting tool maintenance begins with understanding AGMA tool quality standards because this determines the maximum quality that can be maintained. Handling tools with care is fundamental to preserving a tool’s quality. When on a machine, high quality, rigid mounting is necessary to maintain a tool. Lastly, professional tool resharpening ensures top quality maintenance of gear cutting tools.